This competing renewal requests support for the second five year period of a Randomized Clinical Trial (RCT) which is currently in progress. The comparison is essentially between 'one phase' versus 'two phase' orthodontic treatment of children with Class II malocclusion. Data are lacking on the relative merits of these two alternative approaches to this common clinical condition. The current choice is between one stage treatment in the permanent dentition, and a two stage approach involving an initial preadolescent orthopedic phase, followed by definitive treatment in the permanent dentition. The original application proposed to initiate a RCT to meet the short range goals which were (1) to study early partial intervention with respect to effectiveness and cost/benefit, (2) to determine how effectiveness, costs and benefits are modified by the initial malocclusion severity, and (3) to determine whether retention alters the outcome of the early treatment phase. Three hundred and twenty subjects have been enrolled in the RCT. Two hundred and seventy are currently active after three and a half years of this 5 year study. Subjects have been randomized to treatment and retention groups based on initial severity. The progress of these patients is monitored longitudinally for changes and differences in craniofacial morphology, occlusion, functional status, psychosocial parameters, damage to the detention, and treatment time. So far, 105 (39 percent) of the subjects have completed the treatment phase, with approximately 67 percent of the follow up time required. A two phase or dual phase treatment strategy cannot be compared with a single phase strategy unless this RCT is followed to its natural completion. This requires the end of Phase 2 treatment for those receiving dual phase strategy involving subjects headgear or bionator and the end of the single phase treatment period for those randomized to the observation status during he first funding period. Sample sizes suggest there will be sufficient power to detect clinically meaningful differences at the completion of Phase 1 as well as Phase 2 treatment. This application is for the continuation of support to attain completion of the study and is made early, after 3 and one half years, because subjects are completing the first state treatment protocol, and it is necessary to maintain continuity in patient care. This continuation application proposes to (1) finish retention/observation on all subjects and to compare the effectiveness, cost/benefit of the early Phase 1 of a dual phase treatment strategy; (2) determine treatment need and begin phase 2 treatment on subjects exiting he phase 1 protocol; and (3) compare single phase treatment versus a dual phase correction of class II malocclusion in the broadest possible way to produce data on effectiveness and cost (risk) and benefit.